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For some reason, my iMac will not remember that I want to use Preview for all PDF files. It keeps switching to Skitch or something else. I have done the "Get Info", "Open With" and "Change All" several times, but it keeps reverting. Preview is never listed in the right-click "Open With" pull-down menu, but when I use Get Info and select "Other", it's listed as one of the several recommended applications.

Your LaunchServices database may have mild corruption. Try Onyx, go to the Maintenance (tab) -> Rebuilding -> LaunchServices (checkbox). Uncheck all the other items you don't want to do, and click the Run Tasks button.

It may take awhile, reassociating file types with installed applications. Afterwards you may have to choose some desired apps for files again, but the choices should stick.

The reason I ask about Firefox is that, beginning with v57, Firefox picked-up the nasty habit of re-setting the system-level file associations for PDFs and most image formats used on the web (jpg, gif, and png), usually to an app you have on your Mac. In my case, it re-sets PDFs to Acrobat, and image files to Adobe Illustrator. If you go into your Firefox preferences and check your files and applications settings, you'll note that they appear to be correct.

I ran across this right after I updated Firefox to v57 (their new "Quantum" design.) Since the only change I had made to my system was that update, I experimented with down-grading Firefox back to v56 and the problem immediately disappeared. I left it like that for a few days and it didn't return. I then updated Firefox back to v57 and the problem immediately returned. v58 has the same problem. The only fix seems to be either stay at v56 or abandon Firefox.

There's no rhyme or reason when it does it, either. I can go a full day and maybe have to re-set things back to Preview twice. Other days, it's a non-stop battle.

FWIW, I'm on OSX 10.9.5, so it's not exclusive to any particular version of MacOS.